1. Field of the Invention
The present invention relates to an electric starter motor for starting an engine.
2. Description of the Related Art
FIG. 3 is a partial cross-section of a conventional electric starter motor and FIG. 4 is a rear elevation of a conventional electric starter motor.
In FIGS. 3 and 4, the electric starter motor includes: a starter motor 3 for generating torque; a planetary reduction assembly 5 for reducing and outputting the torque from the starter motor 3; an over-running clutch 6 fitted onto an output shaft 4 of the planetary reduction assembly 5; a pinion 7 integrated with the over-running clutch 6 slidably disposed on the output shaft 4; and an electromagnetic switch 9 for controlling the passage of current to the starter motor 3 and for pushing the pinion 7 together with the over-running clutch 6 towards a ring gear 8 of an engine by means of a shift lever 10.
The starter motor 3 includes: a yoke 11 formed into a cylindrical shape having a bottom portion also functioning as an outer frame and a magnetic circuit; permanent magnets 12 disposed at even pitch circumferentially around the inner circumferential surface of the yoke 11; an armature 13 disposed radially inside the permanent magnets 12; a commutator 15 mounted on a rotor shaft 14 of the armature 13; and brushes 17 disposed so as to slide in contact with the commutator 15. A rear bracket 2 fits over the outer circumference of the rear end of the yoke 11, is joined to the yoke 11, and supports the rear end of the rotor shaft 14. Furthermore, a front bracket 1 fits over the outer circumference of the front end of the yoke 11 and is joined to the yoke 11.
The brushes 17 are disposed radially outside the commutator 15 disposed on the rear end of the armature 13, being held in a sidable state by brush holders 16 and constantly pushed against the commutator 15 by brush springs 46.
The planetary reduction assembly 5 is mounted on the front end of the rotor shaft 14, the output shaft 4 thereof being supported by the front bracket 1. The over-running clutch 6 is mounted on the output shaft 4 so as to be able to move in the axial direction and such that rotary motion is transmitted thereto. The shift lever 10 is mounted so as to be able to rotate about an intermediate fulcrum portion 10a, a first end 10b engaging the over-running clutch 6 and a second end 10c being connected to a plunger 20 of the electromagnetic switch 9 mounted in an upper portion of the starter motor 3. Furthermore, packing 18 is fitted into the front bracket 1 so as to support the fulcrum portion 10a of the shift lever 10.
The electromagnetic switch 9 is positioned radially outside the starter motor 3 and the planetary reduction assembly 5, the central axis thereof being disposed generally parallel to the output shaft 4. The electromagnetic switch 9 includes: a switch coil 21 for generating a magnetic force by being supplied an electric current thereto; a frame 22 covering the outside of the switch coil 21 and forming part of a magnetic circuit; a core 23 disposed within the rear end of the switch coil 21 and forming part of the magnetic circuit; the plunger 20 disposed within the switch coil 21 so as to be able to slide freely in the axial direction; a return spring 24 disposed between the core 23 and the plunger 20 to constantly push the plunger 20 towards the front; a hook 25 disposed within the plunger 20 so as to be able to slide freely in the axial direction; a rod 26 disposed at the axial center of the core 23 so as to be able to slide freely in the axial direction; a movable contact 27 mounted on the rear end of the rod 26 for opening and closing the electric current circuit to the starter motor 3; and a pair of fixed contacts 28 disposed opposite the movable contact 27 forming connections to external wiring.
The hook 25 is constantly pushed towards the rear by a lever spring 29. An engaging portion 25a disposed on the end of the hook 25 extending beyond the plunger 20 engages the second end 10c of the shift lever 10. Furthermore, the rod 26 is constantly pushed towards the front by a spring (not shown). In addition, a stopper 31 is mounted on the open end of the plunger 20 to stop the lever spring 29 from dislodging.
Next, the operation of a conventional electric starter motor constructed in the above manner will be explained.
When a key switch (not shown) is closed, current flows through the switch coil 21, whereby the plunger 20 is subjected to the magnetic force generated by the switch coil 21 and is attracted towards the core 23. Consequently, the plunger 20 moves towards the rear (in the direction of arrow A in FIG. 3) against the pushing force of the return spring 24. With the movement of the plunger 20, the hook 25 moves to the rear side. Then, the engaging portion 25a engages the second end 10c of the shift lever 10, and the shift lever 10 rotates counterclockwise in FIG. 3 about the fulcrum portion 10a. The over-running clutch 6 is pressed by the shift lever 10 as it rotates, whereby the over-running clutch 6 and the pinion 7 move together on the output shaft 4 towards the front (in the direction of arrow B in FIG. 3). When the end surface of the pinion 7 comes into contact with the end surface of the ring gear 8, the movement of the over-running clutch 6 and the pinion 7 ceases, but the plunger 20 is attracted and moves further while compressing the lever spring 29 and comes into contact with the rod 26. Then, when the movable contact 27 comes into contact with the fixed contacts 28, the plunger 20 moves further and presses the rod 26 while compressing a spring 30 and stops moving when the end surface of the plunger 20 reaches the end surface of the core 23.
When the movable contact 27 contacts the pair of fixed contacts 28, current flows through lead wires 32, the brushes 17, and the commutator 15 to the armature 13, and the armature 13 rotates. The rotational torque of the armature 13 is transmitted to the output shaft by means of the planetary reduction assembly 5 and the output shaft 4 rotates. At this point, the rotation of the armature 13 is reduced by the planetary reduction assembly 5 and transmitted to the output shaft 4.
With the rotation of the output shaft 4, the pinion 7 also rotates, and when the contact position of the pinion 7 shifts to a position relative to the ring gear 8 where meshing is possible, the over-running clutch 6 and the pinion 7 are pushed out towards the front by the pushing force of the lever spring 29, and the pinion 7 meshes with the ring gear 8. In this manner, the rotational torque of the output shaft 4 is transmitted to the ring gear 8, and the engine is driven.
When the engine has been ignited and the key switch is switched off, the flow of electricity to the switch coil 21 ceases. At that point, the plunger 20 is returned towards the front by the pushing force of the return spring 24 to the state shown in FIG. 3.
In the conventional electric starter motor, because the central axis of the electromagnetic switch 9 is disposed generally parallel to the output shaft 4 radially outside the starter motor 3 and the planetary reduction assembly 5 and is separated by a predetermined distance from the starter motor 3 and the planetary reduction assembly 5 in this manner, one problem has been that the radial dimensions of the electric starter motor have been large, placing significant restrictions on the engine layout during design of the vehicle.
Thus, flattening of the electromagnetic switch has been considered in order to reduce the radial dimensions of the electric starter motor, but in that case one problem has been that special machinery is required, inevitably leading to cost increases.